1. Field of the Invention
The invention relates to an optical subsystem, in particular for a projection exposure system, wherein a light bundle passes through the subsystem and the optical subsystem has at least one optical element, on which the rays of the light bundle impinge on a first used area. Projection exposure systems for microlithography, in particular for wavelengths of ≦193 nm have become known from a plurality of applications. Relative to catadioptric systems, reference is made to DE-A-100 20 592; relative to refractive systems, reference is made to DE 198 55 157 and DE-A-199 05 203, the disclosure content of which is incorporated in its entirety in the present application. The field of the invention includes the field of projection exposure systems, in particular, those that operate with EUV radiation.
2. Description of the Related Art
At the present time, wavelengths in the range of 11–14 nm, in particular 13.5 nm, are discussed as wavelengths for EUV lithography, with a numerical aperture of 0.2–0.3. The image quality in EUV lithography is determined, on the one hand, by the projection objective, and on the other hand, by the illumination system. The illumination system will make available an illumination that is as uniform as possible in the field plane, in which the pattern-bearing mask, the so-called reticle, is disposed. The projection objective images the field plane in an image plane, the so-called wafer plane, in which a light-sensitive object is disposed. Projection exposure systems for EUV lithography are designed with reflective optical elements. The shape of the field in the image plane of an EUV projection exposure system is typically that of an annular field with a high aspect ratio of 2 mm (width)×22–26 mm (arc length). Projection systems are usually operated in scanning mode. With respect to EUV projection exposure systems, reference is made to the following publications:    W. Ulrich, S. Beiersdörfer, H. J. Mann, “Trends in Optical Design of Projection Lenses for UV- and EUV-Lithography” in Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Eds.), Proceedings of SPIE, Vol. 4146 (2000), pages 13–24 and M. Antoni, W. Singer, J. Schultz, J. Wangler. I. Escudero-Sanz, B. Kruizinga, “Illumination Optics Design for EUV-Lithography” in Soft X Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Eds.), Proceedings of SPIE, Vol. 4146 (2000), pages 25–34.
The disclosure content of the above publications is incorporated in its entirety in the present application.
In projection exposure systems, which operate at wavelengths of ≦193 nm, in particular, in the range of ≦157 nm, particularly in the EUV range with wavelengths of <30 nm, the problem arises that radiation in the EUV- or VUV and DU range, respectively, lead to a contamination and/or disruption of the optical surface of the components, which are also denoted optical elements.
The first and last optical surfaces, for example, of refractive systems are particularly at risk of contamination, since these are found in the direct vicinity, e.g., of a source, of a mask, or of a wafer to be exposed. Contaminants are introduced into the optical system by these surfaces. It is thus usual to protect these surfaces on either end, e.g., by a “skin”, which comprises thin foils. Such foils, however, lead to absorption and, in addition, introduce aberrations into the optical system.
The high-energy radiation of the light sources of ≦193 nm leads to the fact that, for example, the residual oxygen components are converted into ozone by the radiation, which in turn attacks and can disrupt the surfaces of the optical elements and their coatings.
In addition, contaminations can be formed on the optical surface due to concentrations of residual gas, such as hydrocarbons from the atmosphere surrounding the optical surface, e.g., due to crystal formation or layers, e.g, of carbon or carbon compounds. Such contamination leads to a reduction in reflection in the case of reflective components and to a reduction in transmission in the case of transmissive components. The contamination may thus depend on the illumination intensity. In the case of EUV lithography, one may use sources, which emit a broadband spectrum. Even after a spectral filtering, e.g., with a grating spectral filter or a zirconium foil, a broad spectrum of high-energy radiation is present. Particularly high is the load in the first optical element up to the first multilayer mirror in an EUV system, since except for the radiation at, e.g., 13.5 nm, the broadband radiation of the source is present and thus the radiation load is maximal. In an EUV projection exposure system, the reflection is particularly reduced due to contamination in the case of the first optical element up to and including the first normal-incidence mirror in an illumination system for a projection exposure system. A normal-incidence mirror in this application is to be understood as a mirror on which the rays of the incident light bundle strike at an angle of <70° relative to the surface normal line.
The reflection loss on the first normal-incidence mirror is greatest in an illumination system of an EUV projection exposure system for this reason, since this mirror receives the highest power density of the light source, but essentially reflects only selectively at 13.5 nm, on account of the multiple layers. All other radiation which is emitted by the EUV source is thus converted into absorption power. Carbon or carbon compounds are again removed by regular cleaning of the mirror, for example, by means of admixtures of argon and oxygen under an RF plasma.
Relative to the cleaning of contaminated optics, reference is made to the following publication:    T. Eggenstein, F. Senf, T. Zeschke, W. Gudat, Cleaning of contaminated XUV-optics at Bessy IIα, Nuclear Instruments and Methods in Physics Research A 467–468 (2001) p. 325–328,the disclosure content of which is incorporated in its entirety in the present application.
Such cleaning of the mirror is, of course, necessary at short time intervals. The useful operating time of the machine is thus very sharply reduced in this way. It may be necessary, for example, to repeatedly clean the first normal-incidence mirror in an EUV projection exposure system as often as approximately every 20 hours of operation. This cleaning lasts, for example, for approximately 2 hours, i.e., 10% of the use time.